The invention relates generally to providing an electrolytic acid solution for electropolishing products made from cobalt-chromium alloys. More particularly, the invention relates to electropolishing medical devices made of cobalt-chromium alloys, and even more particularly, the invention relates to electrolytic acid solutions used to smoothly electropolish stents formed from cobalt-chromium alloy. This invention includes the solutions, the method of electropolishing a cobalt-chromium stent using the solutions, and a cobalt-chromium stent having an outer surface electropolished with the solutions.
While a wide range of products or devices can be made from a cobalt-chromium alloy for use with the present invention, medical devices are particularly suitable due to the biocompatible characteristics of cobalt-chromium products. Thus, for example, implantable medical devices or devices that are used within the human body are particularly suitable and can be made from a cobalt-chromium alloy that has been electropolished in accordance with the present invention. More particularly, and as described in more detail herein, intravascular stents can be made from a cobalt-chromium alloy that has been electropolished according to the invention. Thus, while the description of prior art devices and of the invention herein refers mainly to intravascular stents, the invention is not so limited.
Stents are generally metallic tube shaped intravascular devices which are placed within a blood vessel to structurally hold open the vessel. The device can be used to maintain the patency of a blood vessel immediately after intravascular treatments and can be used to reduce the likelihood of development of restenosis. Expandable stents are frequently used as they may travel in compressed form to the stenotic site generally either crimped onto an inflation balloon or compressed into a containment sheath in a known manner.
Expandable stents formed from metal offer a number of advantages and are widely used. Metallic serpentine stents, for example, not only provide strength and rigidity once implanted they also are designed sufficiently compressible and flexible for traveling through the tortuous pathways of the vessel route prior to arrival at the stenotic site. Additionally, metallic stents may be radiopaque, thus easily visible by radiation illumination techniques such as x-ray film.
It is highly desirable for the surface of the stent to be extremely smooth so that it can be inserted easily and experience low-friction travel through the tortuous vessel pathway prior to implantation. A roughened outer surface may result in increased frictional obstruction during insertion and excess drag during travel to the stenotic site as well as damaging the endothelium lining the vessel wall. A rough surface may cause frictional resistance to such an extent as to prevent travel to desired distal locations. A rough finish may also cause damage to the underlying inflation balloon. A less rough finish decreases thrombogenicity and increases corrosion resistance.
Stents have been formed from various metals including stainless steel, tantalum, titanium, platinum, nickel-titanium which is commonly called nitinol, and alloys formed with cobalt and chromium. Stainless steel has been extensively used to form stents and has often been the material of choice for stent construction. Stainless steel is corrosion resistant, strong, yet may be cut into very thin walled stent patterns.
Cobalt-chromium alloy is a metal that has proven advantages when used in stent applications. Stents made from cobalt-chromium alloy may be thinner and lighter in weight than stents made from other metallic materials, including stainless steel. Cobalt-chromium alloy is also a denser metal than stainless steel. Additionally, cobalt-chromium stents are nontranslucent to certain electromagnetic radiation waves, such as X rays, and, relative to stainless steel stents, provide a higher degree of radiopacity, thus being easier to identify in the body under fluoroscopy.
Metal stents, however, suffer from a number disadvantages. They often require processing to eliminate undesirable burrs, nicks, or sharp ends. Expandable metal stents are frequently formed by use of a laser to cut a framework design from a tube of metal. The tubular stent wall is formed into a lattice arrangement consisting of metal struts with gaps therebetween. Laser cutting, however, typically is at high temperature and often leaves debris and slag material attached to the stent. Such material, if left on a stent, would render the stent unacceptable for implantation. Treatment to remove the slag, burrs, and nicks is therefore required to provide a device suitable for use in a body lumen.
Descaling is typically a first treatment of the surface in preparation for further surface treatment such as electropolishing. Descaling may include, for example, scraping the stent with a diamond file, followed by dipping the stent in hydrochloric acid or a hydrochloric acid mixture, and thereafter cleaning the stent ultrasonically. A successfully descaled metal stent should be substantially slag-free in preparation for subsequent electropolishing.
Further finishing is often accomplished by the well known technique of electropolishing. Grinding, vibration, and tumbling techniques are often not suited to be employed on small detailed parts such as stents.
Prior art electropolishing methods generally improve the smoothness of the metal surface, but as applied to cobalt-chromium stents have had been limited in the ability to produce a desirably consistent smooth surface.
Electropolishing is an electrochemical process by which surface metal is dissolved. Sometimes referred to as “reverse plating,” the electropolishing process actually removes metal from the surface desired to be smoothed. The metal stent is connected to a power supply (the anode) and is immersed in a liquid electrolytic solution along with a metal cathode connected to the negative terminal of the power supply. Current is applied and flows from the stent, causing it to become polarized. The applied current is controlled to control the rate at which the metal ions of the anodic stent are generally removed and diffused through the solution to the cathode.
The rate of the electrochemical reaction is proportional to the current density. The positioning and thickness of the cathode in relation to the stent is important to make available an even distribution of current to the desired portion of the stent sought to be smoothed.
The straightforward application of current, however, does not necessarily translate to even distribution of current across the entire surface sought to be polished. One important feature to creating an even surface on the desired portion of the part is the formation of current differential during the electropolishing process across the surface. Electropolishing provides varied current density to the surface imperfections such as undulations creating protrusions and valleys on the surface. Current density is highest at high points on the surface and lowest at the low points. The increased current density at the raised points causes the metal to dissolve faster at these points thus leveling the surface while forming a corrosion-inhibiting oxide layer.
Electropolishing in the proper electrolytic solution, can serve to smooth out the exposed rough surface to the point where it is ultrasmooth, shiny, and reflective. However, traditional methods have not proved effective to consistently produce an ultrasmooth, shiny finish on a stent comprised of cobalt-chromium alloy.
Treated with traditional electrolytic or etching solutions, such as those specified in ASTM E407-93, ASTM E340-95, and ASTM E1558-93, a stent formed of cobalt-chromium alloy may reveal a variety of finishes ranging from a rather rough and unfinished appearance, a matte finish which is pitted, brown, blackened, feathered, etched, dimpled, rough, and/or uneven.
One prior art electropolishing solution, comprising about 3 parts by volume of about 98% concentrated sulfuric acid and 1 part by volume of about 37% hydrochloric acid (hereinafter referred to as the “3:1” solution), provided an improved surface finish for cobalt-chromium stents. A suitable overall treatment for a cobalt-chromium stent using the 3:1 prior art electropolishing solution includes initial descaling wherein the stents are dipped in a hydrochloric acid solution and thereafter ultrasonically cleaned, electropolishing using the prior art 3:1 solution and then some final electropolishing using a mildly acidic solution. The above treatment produces an acceptable surface finish for a cobalt-chromium stent having a tubular wall with a wall thickness, before surface treatment, of about 0.005 inch. However, it has a tendency to produce an uneven, pitted surface on cobalt-chromium stents having a tubular wall with a wall thickness, before surface treatment, of about 0.004 inch. Heretofore, there has been no effective method to consistently produce an ultrasmooth, shiny finish on the surface of a cobalt-chromium stent with a wall thickness, before surface treatment, of about 0.004 inch.
It would be desirable to provide an electropolishing solution which would acceptably polish stents with wall thicknesses of 0.004 inch as well as up to 0.005 inch. A stent having a 0.004 inch thick tubular wall may be preferable when smaller final dimensions for wall thickness are desired. Also, a stent having a 0.004 inch thick tubular wall will generally have a shorter process time and more uniformity across the length of the stent.
It would also be desirable to provide an electropolishing solution that could acceptably polish a cobalt-chromium stent without an initial descaling treatment or without final electropolishing using a different solution in order to reduce process time and the number of different solutions needed. Further, it would be desirable to provide an electropolishing solution with an increased threshold for both metal ion and water contamination in order to reduce the frequency of changing the solution, thus further reducing process time and costs.
What is needed is a process for treating a product or device made of a cobalt-chromium alloy to consistently produce an ultrasmooth surface as well as to provide a more simplified process, including a reduction in process time, the number of solutions needed and the frequency of replacing the solution. The present invention satisfies these needs.